1. Field of the Invention
This invention relates to an antenna device and a card-shaped storage medium, and more particularly, is suitably applicable to an antenna device and a card-shaped storage medium to write and read out data in non-contact with an integrated circuit (IC) card.
2. Description of the Related Art
IC card systems have been experimentally introduced in fields of ticket checking machines of stations, security systems, and electronic money systems, etc. comprising a data reader/writer and IC cards, which can write/read data to/from IC cards without contact.
Data reader/writer modulates carrier waves based on data to be transmitted, and emits the modulated electromagnetic waves (for example, fluctuating magnetic fields) to an IC card. After receiving the emitted electromagnetic waves, the IC card obtains demodulated data by performing demodulation in a demodulating method corresponding to the modulation method performed by the data reader/writer and processes the data through a predetermined electronic circuit. And the IC card reads out response data from its internal memory based on the data processed through the electronic circuit, modulates the carrier waves based on the read-out response data, and emits the modulated electromagnetic waves toward the data reader/writer.
In such an IC card system, a dipole antenna is provided inside an IC card to receive magnetic energy of electromagnetic waves emitted from a data reader/writer, but it is physically difficult and unrealistic that a dipole antenna is installed inside the IC card. This is because the size of the card itself is extremely small compared with the wavelength of electromagnetic waves.
Because of the circumstances, a loop antenna is installed in an IC card itself to receive electromagnetic waves. In such an IC card with a loop antenna, an electronic circuit is generally made to operate based on the magnetic energy of the electromagnetic waves emitted and received since an internally-installed battery to supply power could hamper miniaturization and weight reduction as well as maintenance-readiness, and in such cases, a loop antenna especially superior in reception efficiency will be indispensable.
As shown in FIG. 1, in the IC card 1 a capacitor 3 is connected with a loop antenna 2 in parallel, and an electronic circuit 4 operates by using electromotive force to be obtained at the both ends of the said capacitor 3.
In this case, the loop antenna 2 converts the change of interlinkage magnetic flux into a voltage under the principle of electromagnetic induction, and supplies this voltage to the electronic circuit 4, and increasing in the area where interlinkage magnetic flux passes can improve reception efficiency in the IC card 1.
In addition, in the IC card 1, a resonant capacitor 3 is connected with the loop antenna 2 in parallel to configure a resonant circuit, so that the reception efficiency may be improved by tuning the circuit to the received electromagnetic waves.
Incidentally, in the IC card 1 having such a configuration, as shown in FIG. 2, the loop antenna 2 and the resonant capacitor 3 efficiently receive electromagnetic waves emitted from a loop antenna 6 of a data reader/writer 5 disposed opposite at a predetermined distance and supplies the received power according to the energy of the said electromagnetic waves to a load resistance Rcir equivalent to the electronic circuit 4.
This IC card 1 is provided with a rectifier circuit 9 composed of a diode 7 and a capacitor 8 at the prestage of the load resistance Rcir. This converts the received power to be supplied to the load resistance Rcir into an easily measurable direct voltage so as to get an accurate measurement.
In this case, the IC card 1 can provide the maximum voltage when the load resistance Rcir is infinitely large, but the current flow is so little that the reception power to be supplied to the electric circuit 4 is very small. Conversely, when the load resistance Rcir is very small, much current flows there, but since little voltage is available, the reception power to be supplied to the electronic circuit 4 is very small. Therefore, in the IC card 1, there is an optimum value of the load resistance Rcir to supply the maximum reception power to the electronic circuit 4.
FIG. 3 shows measurement results of the reception power in the IC card 1, where there is an optimum value of the load resistance Rcir for each distance between the data reader/writer 5 and the IC card 1 (for example, 6 cm, 8 cm, 10 cm and 12 cm) when the maximum value of the reception power P is obtainable from the IC card 1.
That is, the optimum value Rpow of the load resistance Rcir from which the maximum reception power P can be obtained is, for example, approximately 5 kxcexa9 for the distance of 6 cm, approximately 7 kxcexa9 for the distance of 8 cm, approximately 9 kxcexa9 for the distance of 10 cm, and approximately 10 kxcexa9 for the distance of 12 cm.
However, in the IC card 1, the load resistance Rcir in the connected electronic circuit 4 and the optimum value Rpow from which the maximum reception power P can be obtained are not always correspondent. Therefore there still remained such a problem that sufficient reception power can not be obtained and the circuit inside the card did not work normally even if a transmission distance enough to work properly might be given.
In addition, in such a case, the IC card 1 requires an excessive power supply by strong electromagnetic waves from the data reader/writer 5 to keep the circuits operating accurately. This has caused a problem that the power consumption of the data reader/writer 5 is increased.
In view of the foregoing, an object of this invention is to provide an antenna device and a card-shaped storage medium which enables the electric circuit to be connected to always operate efficiently and normally.
The foregoing object and other objects of the invention have been achieved by the provision of an antenna device for supplying a predetermined electronic circuit with electromotive force based on the energy of electromagnetic waves received via the antenna. The antenna device comprises a load resistance conversion means for converting the load resistance of the electronic circuit from the viewpoint of the antenna itself into a predetermined value so as to supply the maximum electromotive force to the electronic circuit with this load resistance conversion means. Thus the electronic circuit connected can always operate efficiently and normally.
Further, the invention provides a card-shaped storage medium for writing/reading data to/from a predetermined electronic circuit via electromagnetic waves in non-contact. The card-shaped storage medium comprises an antenna for receiving electromagnetic waves and supplying a predetermined electronic circuit with electromotive force based on the energy of the electromagnetic waves, and a load resistance conversion means for converting the load resistance of the electronic circuit from the viewpoint of the antenna to a predetermined value, and supplies the maximum electromotive force to the electronic circuit with the load resistance conversion means. Therefore the electronic circuit can always operate efficiently and normally. This reduces the size of the antenna itself and can realize a small tag-type of storage medium of several centimeters square or even several millimeters square, which is much smaller than a normal credit card.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.